Planographic printing plates, such as lithographic plates, have image regions and non-image regions which are essentially co-planar. The image regions are formed from a hydrophobic, oleophilic material to which greasy printing inks are attracted. The non-image regions are formed from a hydrophilic, oleophobic material from which the greasy printing ink is repelled. Thus, on application of the greasy printing ink to the printing plate, the ink is attracted to, and retained on, the image areas and repelled from the non-image areas. The ink can thus be transferred from the printing plate to the printing substrate to produce an image on the printing substrate corresponding to the image areas of the printing plate.
A fount solution is conventionally used to assist in maintaining the hydrophilic properties of the non-image areas and to prevent scumming of the ink into the non-image areas. It is usual to use a polar liquid for this purpose, and water itself may perform satisfactorily as a fount solution for a short time. An aqueous solution including various performance enhancing additives is more commonly used as a fount solution. The performance of the fount solution is optimized to ensure that the solution is repelled by the image areas and is retained on and wets the non-image areas of the plate. Additives may also be used to control the interaction of the fount solution with the ink and the substrate. Known additives include aqueous electrolytes, surfactants and water-soluble polymers.
Various methods have been employed to apply the fount solution to the printing plate. In many conventional lithographic presses, the means used to apply the fount solution (the "dampening system") is entirely separate from the means used to apply the printing ink. In an example of such a method, the fount solution is transferred from a reservoir by a first roller, which is partially immersed in the fount solution, to a second ductor roller. The ductor roller transfers the fount solution (directly or indirectly) to the form rollers which contact the printing plate. The ductor roller oscillates between the first roller and the form rollers (or their precursors) so that contact with each is intermittent, whereby the amount of fount solution which is applied to the plate can be controlled.
In alternative methods, which attempt to avoid transfer of ink from the printing plate to the dampening system, the fount solution is transferred from the first roller to a brush roller. When rotating, the brush roller flicks droplets of the fount solution onto the form rollers or directly onto the printing plate. Similarly, nozzles can be used to spray a line mist of fount solution onto the plate or the form rollers. In another method, the printing plate is contacted only by the inked form rollers. The fount solution must then be transferred from the dampening system to the printing plate via one or more inked rollers.
Each of these alternative methods of application has prolonged direct contact of parts of the lithographic press, such as rollers, nozzles and bearings, with the fount solution. In general, such parts are comprised of metal, especially steel and nickel plated steel, and it has been found that the fount solutions commonly in use are prone to attack these areas of the press, giving rise to corrosion of the various pans. In particular, areas of the press constructed of electroplated nickel are especially vulnerable to attack. The incidence of such corrosion is clearly undesirable, requiring regular and costly replacement of the various pans and, on occasions, resulting in termination of printing runs due to movement of the printing plates on the press, caused by the presence of ill-fitting corroded parts. The corrosion may be associated with any of a number of individual components of the fount solution.
A wide range of corrosion inhibitors have been suggested in the prior art for various applications. Benzimidazole derivatives and various phosphonate and phosphate compounds are typical examples. Many corrosion inhibitors are commercially available under trade names such as Preventol, Cortec, Korantin and Nalco. However, it has previously been :found that many such commercially available corrosion inhibitors are unsuitable or ineffective when used in fount solutions employed on lithographic printing presses.
Corrosion inhibitors typically operate by forming a protective film around the material to be protected. Inevitably, the film will surround the printing plate as well metallic parts of the printing press. Presence of the film on the printing plate, however, causes a reduction in the differential between the hydrophobic image areas and the hydrophilic non-image areas, which differential is the basis of the lithographic printing process. Thus, fount solutions of this type tend to suffer from either uniform excessive ink acceptance, in which case scumming of the background areas becomes apparent; or uniform inadequate ink acceptance, in which case image areas suffer frown "blinding", resulting in poor image quality. In either case, the use of such fount solutions leads to totally unsatisfactory results during printing.
Manufacturers of commercial fount solutions heretofore have attempted to overcome these problems of printing plate performance, while providing corrosion protection for the printing press, by incorporating triazole derivatives such as benzotriazole and tolyltriazole in the fount solution. Whilst the printing results observed with such founts are in some respects satisfactory, the degree of corrosion protection afforded by the inhibitors falls well short of the levels that would be desired. Corrosion of vulnerable parts of the press comprising nickel plated steel, though reduced, is still significant and no protection is afforded to the non-plated steel press parts.
Thus there remains a need for improved fount solutions that enable the printer to produce high quality prints, free from background contamination, while affording a high degree of protection to metal parts of the printing press.